You should untie the bunch of Rotalas (I think the bunch in the front right corner) and plant the stems individually. This is so the lower leaves can get some light. They are also runners, so pretty soon you will get some new shoots coming up to fill up the gaps.
I love your selection of plants - it looks great.
And I agree, a black background will look great. I sticky taped some black cardboard to the back of my tank.
Man, I need to get 1 or 2 more Border Loaches. I have one with 4 Clown Loaches but he doesn't hang out with them. Yours look to be very socialble. I'm going to get another 1 or 2 ASAP.